Exemplary embodiments of the present invention relate to a terahertz continuous wave generator, and more particularly, to a terahertz continuous wave generator which generates a terahertz continuous wave (CW) through photomixing.
Recently, research has been actively conducted on photomixing that uses an optical signal to generate a CW signal having a frequency of nm-wave bands or more, which is not sensitive to low phase noise, temperature, and an operation environment such as a radio environment.
Photomixing is to generate a CW in a ultra-high frequency band, a mm-wave band, and a terahertz (THz) band, which has low phase noise and narrow FWHM (full wave half maximum), by beating two different optical signals through an opto-electric converter (OE converter).
Meanwhile, a terahertz continuous wave generator using photomixing generates two optical signals maintaining a strong correlation and having different wavelengths from one optical source and beats the generated signals, in order to minimize frequency drift and generate a stable CW THz signal.
The terahertz continuous wave generation system, which includes one optical source, passive optical parts, and an OE converter, generates optical signals having two or more kinds of wavelengths from the optical source, and then uses the generated optical signals. Therefore, since the correlation between two wavelengths to be beaten is high, the system may reduce phase noise and increase the frequency stability of the generated signals.
A method for generating a signal of a THz-range frequency from one optical source includes a mode locking laser method, a dual mode laser method, an injection locking method, double sideband-suppressed carrier (DSB-SC) generation, and a frequency comb method.
In the case of the mode locking laser method and the dual mode laser method, an optical device is difficult to manufacture, and the competitiveness thereof is insufficient in terms of the development cost and price of devices. Therefore, the effectiveness of commercialization for the mode locking laser method and the dual mode laser method becomes a problem. In the injection locking method, a locking process should be performed to obtain a desired frequency signal. Therefore, since the operation condition is complicated, the injection locking method has a limit in effectiveness. The frequency comb method includes an optical source, an optical intensity modulator, two optical phase modulators, arrayed waveguide grating (AWG), and an OE converter. In the frequency comb method, the frequency of a generated signal is variable depending on the performance of the optical phase modulator and the AWG, and two expensive optical phase modulators and expensive AWG are required. The DSB-SC generation is a kind of optical heterodyne method and has a simple configuration. Furthermore, it is easy to obtain a desired frequency signal, compared with the above-described methods. Therefore, much research has been conducted on the DSB-SC generation.
A terahertz continuous wave generator using the DSB-SC generation includes one laser diode (LD), passive optical parts, an optical fiber amplifier and so on. The passive optical parts include an optical intensity modulator, an ultrahigh frequency local oscillator (LO), an OE converter, an optical filter and so on, and the optical fiber amplifier is configured to amplify generated DSB signals.
The laser diode is an optical carrier generation source for generating two optical signals having different wavelengths, between which the correlation is high, in the terahertz continuous wave generator using the DSB-SC generation. Such an optical carrier is modulated into DSB signals by a local oscillator signal which is outputted from the ultrahigh frequency local oscillator and then inputted to the optical intensity modulator. The DSB-modulated optical signals are inputted to the OE converter to generate a THz CW signal through photomixing.
In general, an optical filter such as a notch filter or AWG is used to remove an optical carrier existing with the DSB signals on an optical spectrum, in order to increase the stability of the generated THz CW signal and reduce noise. Furthermore, an optical amplifier such as an Er-doped fiber amplifier is used to compensate for attenuation of the DSB signals caused by an insertion loss of the optical filter and increase the magnitude of the DSB signals, thereby generating a THz CW with high power.
The above-described configuration is a related art for helping an understanding of the present invention, and does not mean a related art which is widely known in the technical field to which the present invention belongs.
The conventional terahertz continuous wave generator using the DSB-SC generation may reduce phase noise and increase the frequency stability of a generated signal. However, since an expensive optical source for generating an optical carrier should be used, a manufacturing cost inevitably increases.